1. Field of the Invention
The invention relates to a semiconductor integrated circuit, in particular, a technique for preventing an electrostatic breakdown of a multi-power supply semiconductor integrated circuit.
2. Description of the Related Art
An electrostatic breakdown protection device is provided in a semiconductor integrated circuit in order to prevent a breakdown (an electrostatic breakdown) due to a surge voltage such as a static electricity, an overvoltage, electromagnetic noises emitted from peripheral devices and so on. In particular, as a countermeasure for a case where a surge voltage is applied to a power supply wiring or a ground wiring, an electrostatic breakdown protection diode is connected between the power supply wiring and the ground wiring. Such a semiconductor integrated circuit is described in Japanese Patent Application Publication No. Hei 7-058289.
In recent years, a semiconductor integrated circuit where an analog circuit and a digital circuit are mounted together has been developed. In most of such semiconductor integrated circuits, the power supply voltage of the analog circuit and the power supply voltage of the digital circuit are different. In such a multi-power supply semiconductor integrated circuit, an electrostatic breakdown protection diode is connected between the power supply wiring and the ground wiring in each of the analog circuit and the digital circuit. However, depending on the position of the electrostatic breakdown protection diode, there is a problem that interference due to noises (hereafter, referred to as noise interference) occurs between the analog circuit and the digital circuit, leading to a malfunction or breakdown of the semiconductor integrated circuit.